Microcalorimetric studies on the physical stability of poly-ethylene glycol-grafted liposome

Liposomes were prepared from mixtures of egg-PC, cholesterols and distearoyl-phosphatidyl-ethanolamine covalently attached poly-ethylene glycol (PEG) with a molecular of weight 2000 (DSPE-PEG₂₀₀₀). In this work examines how PEG₂₀₀₀-grafted lipids affect the surface properties of the egg phosphatidylcholine (egg-PC) liposomal bilayer membrane through zeta potential and interaction potential measurements using microcalorimetry. Experimental results demonstrate that the absolute value of the zeta potential of PEG₂₀₀₀-grafted PC liposomes decreased from -19 to -8 mV when increasing DSPE-PEG₂₀₀₀ from 0 to 7 mol fraction, and the repulsive interaction potential of PEG₂₀₀₀-grafted PC liposomes decreased compared with those liposomes without PEG-grafting. However, the phenomenon of fusion between the liposomes incorporated with PEG-grafted PC was reduced. In brief, this result of fusion is contrary to the expectation of the interaction potential measurement; therefore, we believe that the steric hindrance of the grafted PEG₂₀₀₀ molecules on the liposomal surface contribute to a major imposition on the approach between liposomal surface and the formation of inverted micelles which are suggested as the necessary steps of liposome fusion.